const FEE_ENUM = {
  noFee: {
    calcFee: () => 0
  },
  fixedPerLitreFee: {
    calcFee: (litre, opts) => {
      let srvFee = litre * fentoyuan(opts.incFee);
      return srvFee > 2 ? 2 : srvFee;
    }
  },
  percentageFee: {
    calcFee: (disAmt, opts) => disAmt * (opts.pctFee / 100)
  }
};
const selectedStrategy = opts.optFee === 0 || !(disAmt > fentoyuan(opts.thrFee))  ? FEE_ENUM.noFee : opts.optFee === 1 ? FEE_ENUM.fixedPerLitreFee
    : FEE_ENUM.percentageFee;